Is it possible to show glass as being transparent in the viewport
The "Viewport Alpha" settings do not seem to work for the Glass shader the way it works for the Transparent shader (or a mix between Transparent and another shader).  However, if you set the Viewport Alpha to "Add" you will get a sort of fake viewport transparency, which looks about the same for any shader.  Maybe that's good enough for your needs?

Otherwise maybe keep a couple of different shader setups in your material.  Use a simple mix of transparent and glossy (or diffuse, or whatever) for your viewport editing needs.  When it comes time to do a "real" render, switch the inputs to your glass shader.

Edit: here is how these options look in the viewport ("Material" view), as suggested by @LukeD: 



The cube and suzanne to the left have the same material.  It is a glass shader, with Viewport Alpha set to "Add".  The cube and suzanne to the right share a different material.  It is a 50/50 mix of transparent and glossy shaders, with Viewport Alpha set to "Alpha Blend".